Re: NULL terminated strings

[boldt@xxxxxxxxxx]

Joe wrote:
>I'm not a C programmer and I'm a little confused with NULL terminated
>strings. If I see the following line of code, this is supposed to create a
>NULL terminated string, correct?
>
>  C       Eval     SomeFld = SomeText + x'00'
>
>I guess NULL doesn't mean the absence of a value, rather just all zeros.
>
>Where I get confused is the Options(*string) keyword on a procedure.
>Does that automatically append the x'00' onto the end of whatever
>non-blank text is in the field?

Almost.  Yes, it automatically adds the X'00' onto the end of
your string, but all characters, including blanks.  If you
don't want blanks included in your string value, you need to
use %trim or %trimr.

A bit more precisely, options(*string) gives you a pointer to
some temporary storage location that contains your string
value with the x'00' character at the end.

To illustrate, here's the example I posted last week:
------------------------------------------------------------
H dftactgrp(*no) bnddir('QC2LE')
D atol            pr            10i 0 extproc('atol')
D                                 *   value options(*string)
D num             s             10i 0
C                   eval      num = atol('1234')
C     num           dsply
C                   eval      *inlr = *on
------------------------------------------------------------

C function atol expects a null-terminated string as its only
parameter.  Strings in C are often implemented by pointers to
null-terminated strings, thus a pointer value needs to be
passed.  The string passed as parameter in RPG is saved in a
compiler-generated temporary variable, the null is appended to
the end, and the pointer to that location is passed to atol,
which doesn't even realize it's being called by a non-C program.

You can also use null-terminated strings in other ways in RPG.
Let's say you have a pointer to a null-terminated string.  You
can use BIF %str to treat that pointer as an RPG string value.
If pointer variable PTR contains the address of a null
terminated string, %STR(PTR) gives you the value of the string
in RPG character varying format.

Cheers!  Hans

Hans Boldt, ILE RPG Development, IBM Toronto Lab, boldt@ca.ibm.com


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